This invention relates to visual display devices and, more particularly to the uniform illumination of display panels.
There are two general methods of illuminating a display panel. The most common utilizes a single light source arranged so that an acceptable level of light is cast on all viewable areas of the display. This method produces a non-uniform light intensity over the viewing area of the display since the light intensity at a specific point is inversely proportional to the square of the distance between the point and the light source. In cases where the display is fairly compact and all of the viewing areas are approximately the same distance from the light source, the non-uniform lighting is not a serious disadvantage. However, where the display is of irregular shape or of rather long, rectangular shape and where the viewing areas are at substantially varying distances from the light source, the variation in light intensity can be undesirable. To help alleviate this problem a second method of illuminating a display panel was utilized whereby multiple light sources are selectively spaced about the panel in an arrangement that reduces the wide variation in light intensity formed with some of the single light source devices. Here again, however, there may be considerable non-uniformity of light intensity within the viewing field. Additional complexities inherent in this method and resulting increased costs detract from its desirability. Other attempts at solving this problem have centered about the use of "frosted glass" defusers and the like. These attempts have not met with complete success. Another attempt at solving this problem is disclosed in British Pat. No. 1,337,055, Oct. 25, 1972, Bubbins, wherein a single light source is positioned at the focal point of a parabolic shaped reflector which produces a planar field of light of uniform intensity. This method, however, has the disadvantage of being bulky and is difficult to incorporate into existing display designs. The present invention overcomes these difficulties of the prior art by the use of a novel but relatively simple device.